In the manufacture of semiconductor products, substrates such as wafers undergo deposition and etching processes to form features thereon. The processing of semiconductor substrates often leaves residues, such as polymer deposition, between processing steps. Atmospheric inductively coupled plasma torches have been employed to clean substrates in preparation for further processing.
To facilitate discussion, FIG. 1 shows a typical prior art atmospheric inductively coupled plasma torch 100, which includes a double-wall cylinder 102. Cylinder 102 is typically formed out of quartz or a similarly suitable material. A cooling gas inlet 104 permits a cooling gas, such as nitrogen or air for example, to be injected in between the cylinder walls to thermally regulate double-wall cylinder 102 during use. By employing an appropriate cooling gas, thermal damage to atmospheric inductively coupled plasma torch 100 due to the high heat dissipation from the plasma therein is prevented.
A coil 106 is shown wrapped around the outer periphery of double-wall cylinder 102. During use, a process gas (e.g., hydrogen or nitrogen) is introduced into the interior volume of cylinder 102 through process gas inlet 108. When an appropriate driver RF signal (e.g., at 40 MHz) is supplied to coil 106, coil 106 acts as part of a series LC resonance circuit to ignite a plasma from the process gas. To help cool coil 106 during use, liquid cooling is typically employed.
The inductively coupled plasma formed within atmospheric inductively coupled plasma torch 100 is ejected from opening 120. The hot jet of plasma or activated neutral species ejected from opening 120 may then be employed to remove or clean materials, such as unwanted polymer deposition after an ion implantation process, from substrates.
As is known, the induced voltage on coil 106 is a function of the frequency of the driver RF signal. At 40 MHz, a typical atmospheric inductively coupled plasma torch may experience up to 20 KV (peak-to-peak) between the ends of coil 106, for example. The high induced voltage is necessary for igniting plasma at typical atmospheric conditions.
However, the high RF driver frequency employed in the prior art (e.g., 40 MHz or higher) presents cost and engineering challenges. For example, many processing systems already employ lower-frequency RF sources (e.g., 10-30 MHz, such as 13.56 MHz or 27.12 MHz) for etching and deposition. Accordingly, components and expertise for designing, manufacturing, qualifying, and maintaining lower-frequency subsystems are readily available at lower cost. Further, tool-to-tool repeatability is improved when a lower driver RF frequency is employed.
The invention relates to methods and apparatus for igniting and sustaining plasma at a lower driver RF frequency in an atmospheric inductively coupled plasma torch.